1. Field of the Invention
This invention relates to a coke drum, and more particularly, to a discharge system for a coke drum.
2. Background of the Invention
In a conventional delayed coking process, petroleum residues are fed at elevated temperatures to a large steel vessel called a coke drum, where the residues are thermally cracked and formed into coke. When the coke drum has filled with coke, the drum is injected with steam and cooled with water. Subsequently, top and bottom covers of the drum are removed in a process called unheading. Despite the open bottom of the drum, however, the coke often remains inside the drum until it is cut. When cut, the coke falls out of the open bottom of the drum down a chute or other conduit, so that it may be transported and further processed.
As discussed above, the conventional coking process requires unheading the coke drum. Since the coke drum must contain a severe atmosphere of elevated temperatures, the bottom cover is typically secured to the coke drum by dozens of bolts, which often must be loosened manually. As a result, unheading is typically a labor intensive chore.
A further drawback of the conventional unheading process is that it is difficult to use in the production of shot coke. Shot coke is unique because it will not always remain in the drum during and after unheading. As a result, the coke will often pour out of the drum as the bottom cover is being removed. In addition, the shot coke may rest on the bottom cover, putting a large load on the bottom cover and making its controlled removal difficult.
Examples of apparatuses designed to mitigate the difficulties of unheading the coke drum are shown in U.S. Pat. No. 2,318,131 to Utterback and U.S. Pat. No. 2,702,269 to Geller. Both Utterback and Geller disclose continuous processes that do not require unheading.
Utterback shows a spray coking process, in which spray-coked material is formed in chamber 1 and falls through a duct 2 into a vessel 3, which contains oil. The solid, spray-coked material is suspended in the oil, and the resultant mixture is pumped by a pump 6 into separators 8 and 9, where screens collect the coke. Oil is pressured out of the separators 8 and 9, and the collected coke is partially dried using steam. The coke then is removed through discharge piping 24, which is shown to contain a valve. The oil is returned to the vessel 3 via valved piping 11 or to an upstream tank 16 via valved piping 23.
Although Utterback mitigates the unheading problem, it also suffers from drawbacks. For example, rather than producing ready-to-transport coke out of the coke drum 1, Utterback requires additional components (such as a vessel full of oil, a pump and separators) to process the coke and get it ready for transport. In contrast, a batch process produces coke that is ready for transport directly out of the coke drum.
Geller also discloses a continuous coking process. In Geller, feed material for coking is routed through pipe 2 into a coking space 1 via a nozzle 3 directed vertically upwards. A casing 8 and lining 9 surround the coking space 1 for indirect heating. The coke falls onto an outlet branch 21 and is discharged from the bottom of the vessel using a worm 22, which is self-sealing against the passage of gas.
Although Geller's continuous process does not require unheading, it does suffer from at least two drawbacks. First, the coke leaving the drum is at a high enough temperature that it may combust if it comes into contact with air, so additional components are required to cool the coke in an oxygen-depleted environment. Second, the coke drum needs an outer casing to heat its walls, implicating increased cost for installation and manufacture.
As the drawbacks of Utterback and Geller indicate, the continuous process for producing coke is not always desirable. Therefore, there is a need for an apparatus to discharge coke from a coke drum in batches, but without the drawback in conventional batch cokers of having to unhead the coke drum each time coke is removed.